The point in common of most approaches of the prior art of the field consists in methods aiming at improving strains with known genetic heritage and/or constructed genetic heritage and the capabilities of which for producing ethanol are generally studied in media and under <<ideal>> laboratory conditions.
Indeed, scientific literature as well as patent documents analyzed by the Applicant most often teach methods for obtaining haploid or diploid strains, little tolerant to stresses notably to strong concentrations of ethanol and/or to high temperatures and/or to fermentation inhibitors. Further, these methods for the most part require resorting for these strains to the use of auxotrophy markers and/or markers of resistance to antibiotics which may disqualify them for subsequent use in an industrial medium for obvious reasons of cost or even sometimes of health or respect of the environment.
The growth properties of strains developed previously are generally insufficient and these strains have never been confronted with biomass production requirements on an industrial scale, i.e. in order to only mention three of them: strong growth rate, drying capacity, storage stability.
If so-called fermentative performances (anaerobic ethanol production capacity) are obtained in synthetic or defined media, so-called laboratory media with these previous strains, they generally cannot be transposed in industrial media including complex mixtures for example stemming from cellulose-processing residues which contain toxic compounds which may inhibit at different levels the yeast's cell mechanism, notably furfural, HMF, phenolic derivatives, acetic acid. Further, the <<scale up>> or scale transposition capacity of these earlier ethanol production methods is seldom documented.
The document WO 2008/133665 teaches the production of alcohol from a yeast strain with a <<genetic background>> of the type:                Mutated STP15 gene (F117S, Y195H, K218R).        Exogenous genes coding for XI/XR/XDH or XK.        
“XI” means xylose isomerase, “XR” means xylose reductase, “XDH” means xylitol dehydrogenase, and “XK” means D-xylulokinase.
Document WO 2005/113774 describes a recombinant operon comprising two nucleic acid sequences respectively coding for an XI of E. coli and an XDH of Trichoderma reesei in the context of the production of xylitol.
The document PloS Genetics of Gavin Sherlok et al., published on May 13, 2010, describes an XDH1 gene which is present in some specific Saccharomyces cerevisiae strains, which may code for a xylitol dehydrogenase.
The document in the name of David Brat, Eckard Boles and Beate Wiedemannn, in Appl. Environ. Microbiol., April 2009, Vol. 75, No. 8, p. 2304-2311, describes the expression of the xylose isomerase gene from Clostridium phytofermentans in Saccharomyces cerevisiae. 
It emerges from this review of the documents of the prior art, as well as of the work of the inventors of the present invention, that given the very different genetic background/heritages of the strains of Saccharomyces cerevisiae yeasts applied with the purpose of growing on and/or fermenting xylose, the consequences for example of overexpression and/or of deletion of native genes and/or of the introduction of one or more heterologous genes cannot be predicted.